cpsw: rename register banks to match the reference manual

The code mixes up the CPSW_SS and the CPSW_WR register naming. This patch
changes the names to conform to the published Technical Reference Manual
from TI, in order to make working on the code less confusing.

Signed-off-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>